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- String (String str):
- Constructor: Creates a new String object with the same characters as the given String.
- Example:
String s1 = "Hello";
String s2 = new String(s1); // s2 now has the value "Hello"- int length():
- The
length()method returns the number of characters in aString. - Example:
String message = "Hello, Java";
int len = message.length(); // len will be 10- String substring(int from, int to):
- The
substring(from, to)method extracts a portion of the originalStringfrom indexfrom(inclusive) to indexto(exclusive). - Example:
String original = "Programming";
String sub = original.substring(3, 7); // sub will be "gram"- String substring(int from):
- The
substring(from)method extracts a portion of the originalStringfrom indexfromto the end. - Example:
String original = "Copilot";
String sub = original.substring(2); // sub will be "pilot"- int indexOf(String str):
- The
indexOf(str)method returns the index of the first occurrence ofstrin theString, or -1 if not found. - Example:
String sentence = "This is a test sentence.";
int index = sentence.indexOf("test"); // index will be 10- boolean equals(String other):
- The
equals(other)method checks if the currentStringis equal to theotherString. - Example:
String name1 = "Alice";
String name2 = "Alice";
boolean isEqual = name1.equals(name2); // isEqual will be true- int compareTo(String other):
- The
compareTo(other)method compares twoStringlexicographically. - Example:
String word1 = "apple";
String word2 = "banana";
int result = word1.compareTo(word2); // result will be negative- Integer(int value):
- The
Integerclass wraps anintvalue into an object. - Example:
Integer num = new Integer(42);- Integer.MIN_VALUE:
- Represents the minimum value that an
intcan hold (-2147483648).
- Integer.MAX_VALUE:
- Represents the maximum value that an
intcan hold (2147483647).
- int intValue():
- Converts an
Integerobject to its correspondingintvalue. - Example:
Integer num = new Integer(123);
int value = num.intValue(); // value will be 123- Double(double value):
- The
Doubleclass wraps adoublevalue into an object. - Example:
Double pi = new Double(3.14159);- double doubleValue():
- Converts a
Doubleobject to its correspondingdoublevalue. - Example:
Double height = new Double(175.5);
double heightValue = height.doubleValue(); // heightValue will be 175.5- static int abs(int x):
- Returns the absolute value of an integer
x. - Example:
int result = Math.abs(-5); // result will be 5- static double abs(double x):
- Returns the absolute value of a double
x. - Example:
double result = Math.abs(-3.14); // result will be 3.14- static double pow(double base, double exponent):
- Computes
baseraised to the power ofexponent. - Example:
double result = Math.pow(2, 3); // result will be 8.0- static double sqrt(double x):
- Computes the square root of a double
x. - Example:
double result = Math.sqrt(25); // result will be 5.0- static double random():
- The
random()method generates a random double value between 0.0 (inclusive) and 1.0 (exclusive). It is part of theMathclass in Java. - Example:
double randomValue = Math.random(); // Generates a random value between 0.0 and 1.0- int size():
- The
size()method is commonly used in collections (such as lists, sets, and maps) to determine the number of elements in the collection. - Example (using a list):
List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
int listSize = names.size(); // listSize will be 2- boolean add(E obj):
- The
add(obj)method adds an elementobjto a collection (e.g., a list or set). It returnstrueif the addition is successful. - Example (using a list):
List<Integer> numbers = new ArrayList<>();
boolean added = numbers.add(42); // added will be true- void add(int index, E obj):
- The
add(index, obj)method inserts an elementobjat the specifiedindexin a list. - Example (using a list):
List<String> fruits = new ArrayList<>();
fruits.add("apple");
fruits.add(1, "banana"); // Inserts "banana" at index 1- E get(int index):
- The
get(index)method retrieves the element at the specifiedindexfrom a list. - Example (using a list):
List<String> colors = Arrays.asList("red", "green", "blue");
String thirdColor = colors.get(2); // thirdColor will be "blue"- E set(int index, E obj):
- The
set(index, obj)method replaces the element at the specifiedindexwith the new elementobjin a list. - Example (using a list):
List<Integer> scores = new ArrayList<>();
scores.add(80);
scores.set(0, 90); // Replaces the score at index 0 with 90- E remove(int index):
- The
remove(index)method removes the element at the specifiedindexfrom a list. - Example (using a list):
List<String> animals = new ArrayList<>();
animals.add("cat");
animals.add("dog");
String removedAnimal = animals.remove(1); // Removes "dog" from the list- boolean equals(Object other):
- The
equals(other)method checks if the current object is equal to theotherobject. It is commonly used for comparing strings. - Example:
String name1 = "Alice";
String name2 = "Alice";
boolean isEqual = name1.equals(name2); // isEqual will be true- String toString():
The
toString() method in Java is a member of the Object class. Since it is inherited by all Java classes, you can override it to provide a string representation of any Java object. This method is very useful when you need a human-readable description of an object, typically for debugging or logging purposes.Here's a breakdown of its uses and how to implement it:
Default Implementation
By default, the
toString() method returns a string that consists of the class name followed by the "@" symbol and then the hexadecimal representation of the object's hash code. This default implementation is rarely useful in practical applications.Overriding toString()
You can override the
toString() method in any class to return a more informative, readable string that describes an instance of that class. This is particularly useful for debugging, as you can print out objects directly and get meaningful information.Example
Here's a simple example of overriding the
toString() method in a Person class:In this example, the
toString() method is overridden to return a string that includes the person's name and age. When you print an instance of Person, the toString() method is called automatically, resulting in a more informative output.Codes：
- 作者:现代数学启蒙
- 链接:https://www.math1234567.com/article/javareference
- 声明:本文采用 CC BY-NC-SA 4.0 许可协议，转载请注明出处。
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